Process for the manufacture of monomethylhydrazine



Jan. 21, 1969 R. E. BAILEY 3,423,464

PROCESS FOR THE MANUFAQTURE OF MONOMETHYLHYDRAZINE Filed Aug. 25, 1966 INVENTOR. RANDAL E. BAILEY BY TZ $5197,

A T TORNEY United States Patent 2 Claims This invention relates to an azeotropic mixture of azomethane and monomethylamine. This invent-ion also relates to an improvement in the manufacture of monomethylhydrazine from monomethylamine and chloramine based on the discovery of the azeotropic mixture of azomethane and monomethylamine.

Monomethylhydrazine is a well known liquid propellant. It is commercially prepared by reacting chloramine with an excess of monomethylamine in an equeous solution as described by Knight, Petroleum Refiner, February 1962, pages 179-184. In addition to the desired monomethylhydrazine, the aqueous reaction mixture also contains alkali metal chloride, unreacted monomethylamine, ammonia, nitrogen and azomethane. Azomethane is an explosive impurity which must be removed from the reaction mixture in the commercial process wherein unreacted monomethylamine is recycled after isolation of the monomethylhydrazine by conventional techniques. Although the amount of azomethane produced in the reaction of chloramine with monomethylamine is small, usually from 1 to 1000 ppm. of the reaction mixture, it accumulates in the monomethylamine stream and presents a hazard because of its explosive potential.

In the previously-described process for the preparation of monomethylhydrazine, fixed gases such as nitrogen are first separated from the reaction mixture. The reaction mixture is then distilled at substantially atmospheric pressure up to about 3340 mm. Hg to separate it into a first overhead stream containing substantially all of the azomethane, monomethylamine and ammonia. A first bottoms stream is produced comprising substantially all of the monomethylhydrazine, water and alkali metal chloride. Since azomethane is an explosive material, separation of the pure compound by distillation is not feasible in the commercial operation, wherein stringent removal and disposel condition would be impractical.

Pending US. patent application Ser. No. 378,012, filed on June 25, 1964, teaches that removal of azomethane from the recycle stream is accomplished by adding large amounts of water to the azomethane-monomethylamineammonia mixture and subsequently removing a mixture of water and azomethane overhead by fractional distillation. Although accomplishing the desired result, it is apparent that the addition and subsequent removal of large amounts of water reduces effective plant capacity and is therefore commercially unattractive.

Now it has been found that azomethane and monomethylamine form a constant boiling azeotropic mixture of the low-boiling type which is readily removed from a substantially anhydrous azomethane-monomethylamine mixture by fractional distillation. The azeotropic mixture consists of about mole percent azomethane and about 90 mole percent monomethylamine, and boils at a temperature of approximately -6.7 C. at atmospheric pressure. This discovery results in a simplified, economical 'ice process for preparing commercial quantities of monomethylhydrazine characterized by a significantly improved safety feature. Since the concentration of azomethane in the azeotrope is below the explosive limits of the compound, no safety precautions are necessary in the process and the azeotrope is easily removed by the use of conventional process equipment.

The azeotropic composition of this invention is provided by fractionally distilling at atmospheric pressure a solution of azomethane and monomethylamine. While the composition and boiling point of the azeotrope is referred to in the claims and specification herein with respect to atmospheric pressure, it is understood that the composition and boiling point will vary with pressure, and hence, the azeotrope can be obtained from suitable solu tions at both reduced and elevated pressures.

Attached FIGURE 1 is a flow sheet of a preferred embodiment of the improved process of this invention. Monomethylhydrazine (MMH) synthesis liquor is prepared in synthesis reactor 1, substantially as described by Knight in the publication cited above from monomethylamine (MMA) and aqueous chloramine. The synthesis liquor passes via line 11 into surge tank 2 acting as a separator for removing overhead fixed gases including largely nitrogen together with some MMA and ammonia. The liquid effluent from surge tank 2 passes via line 12 to distilling column 3. The feed liquor in line 12 contains dissolved MMH, MMA, ammonia, by-product AZM, water and salt. Overhead from column 3 via 13 a stream of volatiles including MMA, AZM and ammonia is fed to distilling column 4. The bottoms from column 3 contain the MMH as the principal product together with water and salt. These pass via line 21 to further processing by known means for the recovery of more concentrated or anhydrous MMH. Ammonia is removed overhead from column 4 by distillation. The bottoms from column 4 containing MMA and AZM are fed via line 14 to column 5. An azeotrope of AZM and MMA is removed overhead from column 5 by distillation. The bottoms from column 5, consisting essentially of MMA, is recycled to synthesis reactor 1 via line 15.

The following examples illustrate the practice of this invention.

Example 1 In a system essentially similar to that shown in FIG- URE 1, the feed of synthesis liquor to the surge tank 2 amounted to 69 g.p.m. (gallons per minute) and contained mostly water, salt and about 0.6 percent of MMH together with byproducts including nitrogen, AZM, excess MMA and ammonia. The gaseous components were removed from the synthesis liquor in the surge tank and the bottoms passed to column 3. The overhead stream from column 3, operated at 2050 mm. Hg and a bottoms temperature of C. amounted to about 15 g.p.m. and contained substantially all of the unreacted MMA and ammonia togther with the AZM. This overhead stream was passed into column 4. The aqueous effluent from the bottom of column 3 contained substantially all of the MMH product. This efliuent was charged at the rate of about 56 g.p.m. to further processing for anhydrous MMH. Ammonia was removed overhead from column 4 and the bottoms, consisting of AZM and MMA passed into distillation column 5. Column 5 contained the equivalent of 40 theoretical plates; the feed was introduced on the 20th plate from the top of the column. The column was operated at atmospheric pressure with a reflux ratio of 100:1.

A sample of the distillate was removed at a vapor temperature of -6.7 C. and analyzed by conventional titration procedures. Azomethane content was determined by titrating an aqueous solution of part of the sample with KIO A second solution of the distillate in water was reacted with CS in an isopropanol-pyridine medium and the reaction product titrated with NaOH to determine the number of milliequivalents of MMA. From this data it was determined that the distillate contained about mole percent AZM and about 90 mole percent MMA.

The bottoms from column 5, containing MMA and .0001 mole fraction AZM, were recycled to synthesis reactor 1.

Example 2 For purposes of confirmation of the azeotropic composition determined in Example 1, a synthetic mixture of azomethane and monomethylamine was prepared and distilled. Azomethane was provided by reacting monomethylhydrazine and formaldehyde to obtain methylene monomethylhydrazine as described in Chem. Ber. 90, 1299302 and 1307-9 (1957), and then hydrolyzing the methylene monomethylhydrazine with aqueous sodium hydroxide. The gaseous reaction products, comprising water and ammethane, were passed through a drying tube to remove the water and the gaseous anhydrous azomethane passed into a stainless steel bomb containing monomethylamine which had been cooled to 40 to -50 C. with Dry Ice.

The resulting mixture of azomethane and monomethylamine was pressured with nitrogen from the steel bomb into a glass still as described by Gillespie, D. T., Ind., Eng. Chem, Anal. Ed, 18, 575 (1946), which was modified by jacketing the recirculating lines to maintain the temperatures employed in the process.

The boiling chamber of the still was heated at atmospheric pressure to equilibrium (-6.6" 0.). Samples of the liquid and vapor condensate were withdrawn, absorbed in known quantities of water, and titrated in accordance with the procedure described in Example 1. It was determined that the liquid contained 6.83 mole percent azomethane and 93.17 mole percent monomethylamine and the vapor 8.79 and 91.21 mole percent respectively.

Several additional distillations and analyses were carried out in the same manner, using monomethylamine-a20- methane mixtures prepared as previously described. The results are tabulated below:

Absolute pressure Temp. Mole percent azomethane (mm. Hg.) C.)

Vapor Liquid The concentration of azomethane in the vapor was plotted vs. the concentration in the liquid. A line was drawn at a 45 angle to each axis, representing all compositions at which the vapor concentration is equal to the liquid concentration. The azeotropic composition, determined from the point where the 45 line crossed the vaporliquid curve for the experimental data, was 10 mole percent azomethane, mole percent monomethylamine at 760 mm. Hg and 6.7 C. The pressure and temperature are averages for the experimental data.

What is claimed is:

1. In a process for the manufacture of monomethylhydrazine wherein a synthesis liquor is prepared by reacting chlorarnine and excess monomethylamine in aqueous solution and said synthesis liquor is fractionally distilled to form a first overhead stream consisting essentially of ammonia, monomethylamine and azomethane, and a first bottoms stream comprising water and monomethylhydrazine, the improvement which comprises removing from said first overhead stream by fractional distillation an azeotropic mixture of azomethane and monomethylamine.

2. The process of claim 1 wherein the monomethylamine remaining after the removal of said azeotropic mixture of azomethane and monomethylamine is recycled to the synthesis reactor.

References Cited UNITED STATES PATENTS 2,806,851 9/1957 Sisler et al 260-583 X CHARLES E. PARKER, Primary Examiner.

R. L. RAYMOND, Assistant.Examiner.

US. Cl. X.R. 252182, 162 

1. IN A PROCESS FOR THE MANUFACTURE OF MONOMETHYLHYDRAZINE WHEREIN A SYNTHESIS LQUOR IS PREPARED BY REACTING CHLORAMINE AND EXCESS MONOMETHYLAMINE IN AQUEOUS SOLUTION AND SAID SYNTHESES LIQUOR IS FRACTIONALLY DISTILLED TO FORM A FIRST OVERHEAD STREAM CONSISTING ESSENTIALLY OF AMMONIA, MONOMETHYLAMINE AND AZOMETHANE, AND A FIRST BOTTOMS STREAM COMPRISING WATER AND MONOMETHYLHYDRAZINE, THE IMPROVEMENT WHICH COMPRISES REMOVING FROM SAID FIRST OVERHEAD STREAM BY FRACTIONAL DISTILLATION AN AZEOTROPIC MIXTURE OF AZOMETHANE AND MONOMETHYLAMINE. 